This study will determine the disposition of theophylline in asthmatic children receiving chronic theophylline therapy who are in status asthmaticus, undergoing a febrile illness or receiving concurrent corticosteroid therapy. These specific situations present a problem in the management of the asthmatic child on theophylline. The pharmacokinetics of theophylline will be determined in each of the above mentioned clinical situations. The metabolites of theophylline will be identified and the pharmacokinetics of these metabolites will be determined. Analysis of this data will indicate any changes in the clearance or metabolism of theophylline in asthmatic children undergoing these relevant clinical situations. Theophylline and metabolite concentrations will be determined by high pressure liquid chromatography. Metabolites will be identified by high pressure liquid chromatography with final verification by mass spectroscopy. Computer analysis will be used to determine the relevant pharmacokinetic parameters. The data obtained from these studies will allow the prediction of theophylline disposition in asthmatic children who are in status asthmaticus, undergoing a febrile illness, or receiving concurrent corticosteroid therapy. Guidelines for theophylline dosage in these situations will be developed in order to avoid theophylline toxicity or inadequate drug levels causing lack of control of clinical symptoms. It is anticipated that these specific guidelines will reduce the need for frequent determinations of theophylline levels in these children while preserving optimal care.